The invention relates to the field of integrated circuit manufacturing, and in particular to a technique for processing a silicon wafer to provide a hydrophobic surface.
During the manufacturing of integrated circuits, silicon wafers are prepared for thermal diffusion and oxidation operations. The preparation includes cleaning the wafer to remove sub-micron surface contamination. This often includes first immersing the wafer in an alkaline mixture of deionized (DI) water, hydrogen peroxide and ammonium hydroxide to remove light organic residues. In this step, the solution may generally be in the range of 5:1:1 to 7:2:1, and be heated to 75-85xc2x0 C. The wafer is then immersed in DI water to rinse out the chemicals as well as particulate contaminants from its surface. Following the DI water rinse, the wafer is processed in an acidic mixture (e.g., 5:1:1 to 7:2:1 DI water, hydrogen chloride, and hydrogen peroxide solution) to remove trace metallic impurities. The wafers are then rinsed again in DI water. As known, this process is often referred to as a RCA clean chemistry.
A limitation of this technique is that a silicon wafer, when immersed in the hydrogen peroxide solution, produces an oxide film on its surface. To remove this oxide film from the wafer surface, the silicon wafer is typically processed in a hydrofluoric acid solution that has been diluted with DI water. At this stage of processing, the wafer has a relatively good hydrophobic surface, but if one were to simply remove the wafer following the immersion in the hydrofluoric acid solution, there is a significant safety issue since hydrofluoric acid is extremely aggressive on bone, and thus dangerous to humans. Therefore, the wafer needs to be rinsed again in DI water prior to handling. However, DI water is essentially an oxidizing medium that undesirably creates oxides on the surface of the wafer. Therefore, in order to obtain a relatively good hydrophobic surface, handling of the wafer immediately following immersion in the hydrofluoric acid solution is required, regardless of its safety and handling issues.
Therefore, there is a need for developing a new cleaning technique that provides a silicon wafer with a hydrogen-terminated hydrophobic surface. Further, this new technique needs to be capable of producing a hydrophobic silicon wafer with no hydrofluoric acid solution on its surface so the wafer can be safely handled.
Briefly, according to an aspect the present invention, at least one silicon wafer is loaded into a single closed vessel that contains a first solution of ammonium hydroxide, hydrogen peroxide solution and DI water. This first solution is removed from the vessel and DI water is then introduced into the vessel until the wafer is completely immersed. The DI water is removed from the vessel. Then, a second solution of hydrofluoric acid, hydrochloric acid solution and DI water is added to the vessel to fully immerse the wafer. The second solution is then removed from the vessel, and as the second solution is slowly being drained from the vessel, an alcohol solution in a hot and low-pressure nitrogen carrier gas is introduced to the vessel. As the level of the liquid within the vessel is falling below the height of the wafer within the vessel, the wafer is dried.
Advantageously, the technique of the present invention occurs in a single and closed vessel. The cleaning process is in-situ. In addition, the thin coating of alcohol solvent on the wafer results in a lower surface tension which facilitates ensuring that the surface of the wafer is hydrogen terminated, thus hydrophobic.
These and other objects, features and advantages of the present invention will become apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.